This invention relates to an optical recording medium which is capable of accomplishing recording and reproduction by a light beam.
As an optical recording medium and in an optical disc or the recording layer thereof, there are known thin films of rare earth-transition metal alloys, thin films of reductive oxides such as chalcogen compounds utilizing the phase transition from non-crystalloid to crystalloid, heat mode recording mediums, thermoplastic recording mediums, etc. For example, as opto-magnetic recording mediums formed of thin films of rare earth-transition metal alloys, there are known polycrystal thin films such as MnBi, MnCuBi and the like, amorphous thin films such as GdCo, GdFe, TbFe, DyFe, GdTbFe, TbDyFe and the like, and single crystal thin films such as GdIG and the like.
Among these thin films, the amorphous thin films have recently been considered to be excellent as an opto-magnetic recording medium by taking into account the film formability when a thin film of large area is made at a temperature in the vicinity of room temperature, the writing efficiency for writing signals with small light-heat energy and the reading-out efficiency for reading out the written signals at a good S/N ratio. Among them, GdTbFe which has a great Kerr rotation angle and has a curie point of about 150.degree. C. is suitable as an opto-magnetic recording medium. Further, the inventors have studied with a view to improve the Kerr rotation angle and as a result, have found that GdTbFeCo is an opto-magnetic recording medium having a sufficiently great Kerr rotation angle and enabling reading-out at a good S/N ratio to be accomplished.
Generally, however, amorphous magnetic materials such as GdTbFe, etc. used in magnetic recording mediums including opto-magnetic recording mediums have a drawback that they are poor in corrosion resistance. That is, if these materials contact the atmosphere or steam, their magnetic characteristic is reduced and finally, they become completely oxidized and transparent. This problem is common to the opto-magnetic recording mediums and the aforementioned optical recording mediums.
In order to eliminate such a drawback, it has heretofore been proposed to provide a protective cover of transparent substance, for example, a protective layer of SiO.sub.2 or SiO, on a recording layer, or to adopt an air sandwich structure in which the recording layer is enveloped by inactivated gas or a cemented structure in which a substrate is further provided on the protective layer with an adhesive agent interposed therebetween, but practically sufficient corrosion resistance has not been obtained.
It is an object of the present invention to provide an optical recording medium having its corrosion resistance improved without its characteristic as a recording medium being marred.
The above object of the present invention is achieved by forming, in an optical recording medium having an optical recording layer provided on a substrate, on one or each side of said optical recording layer, a carbide film of a substance selected from among tungsten, boron, molybdenum, hafnium, niobium, titanium, chromium, vanadium, tantalum and silicon.
The carbide film as described above is formed by the evaporation method or the sputtering method. Particularly, the high frequency (RF) sputtering is suitable for the formation of hafnium carbide film, vanadium carbide film and tantalum carbide film, and the electron beam evaporation or the RF sputtering is suitable for the formation of tungsten carbide film, boron carbide film, molybdenum carbide film and niobium carbide film. Further, the reactive evaporation or the reactive RF sputtering is suitable for the formation of titanium carbide film and chronium carbide film, and the RF sputtering is suitable for the formation of silicon carbide film.
In the present invention, where a carbide film is to be formed subsequently to the optical recording layer, it is desirable to form the recording layer by a method such as sputtering, and thereafter continuously form a carbide film without breaking the vacuum. Also, where a carbide film is to be formed on a substrate and an optical recording layer is to be formed thereon and further a carbide film is to be formed thereon, it is preferable to form the films continuously in the same tank without breaking the vacuum.
It is another object of the present invention to provide an optical recording medium which is great in the S/N ratio of read-out signal moreover has a sufficient corrosion resistance and has a structure in which an optical interference film, a reflecting layer, etc. are laminated in succession from the recording layer side to obtain, especially, the enhancement effect of the magneto-optical effect.
The above another object of the present invention is achieved by an optical recording medium provided with at least an optical recording layer and a reflecting layer on a substrate, characterized in that an optical interference film consisting of silicon carbide is provided between said optical recording layer and said reflecting layer.
It is a still another object of the present invention to provide an optical recording medium characterized in that at least an optical recording layer and a film of carbide are provided on a substrate having an SiO film as an under-laid layer provided thereon in contact therewith.
As the film of carbide, use may be made of any carbide having a transmitting property required with respect to the light used and chemically stable and excellent in moisture resistance, and preferred carbides are SiC and B.sub.4 C. The film thickness of carbide may preferably in the range of 200 .ANG. to 2000 .ANG.. If the film thickness is less than 200 .ANG., the film will be insufficient in corrosion resistance, and if the film thickness exceeds 2000 .ANG., there will be the undesirable possibility of the recording sensitivity being reduced or cracks or the like being created in the film.
The optical recording medium of the present invention may assume any of the cemented structure and the air sandwich structure.
The SiO film provided as an under-laid layer is provided to improve the unitability of the film provided thereon with the substrate, and the thickness of that film may preferably in the range of 500 .ANG. to 2000 .ANG..